1. Field of the Invention
This invention relates to a focusing apparatus for a zoom lens and, more particularly, to a focusing apparatus suitable for zoom lenses of an inner or rear focus system based on displacement of some lens units in a lens system and also of an object side lens feed system based on displacement of object side lens units or a whole lens feed system based on displacement of whole lens units.
2. Related Background Art
As a focusing system for a zoom lens, there are those of a so-called object side lens feed system, in which object side lens units nearest to the object side are displaced, an inner or rear focus system, in which some lens units in a lens system are displaced, and of a whole lens feed system, in which the whole lens system is displaced for focusing. Usually, in the inner or rear focus system and in the whole lens feed system, the necessary extent of feed for the same object distance is changed with changes in the focal distance of the overall lens system. Therefore, even when focusing is done with an object distance corresponding to a given focal distance, by changing the focal distance through zooming the position of focus is greatly changed. This means that the focusing should be done every time zooming is done.
There are proposed various methods of automatically and electrically effecting the focusing of lens. These methods may be utilized for auto-focus systems but can not be utilized for a commonly termed manual focus system, in which the focusing is done by manually operating a distance adjustment ring (or focus ring).
Further, even in case of application to an auto-focus system a time is necessary for calculating the extent of feed necessary for zooming every time zooming is done, thus posing problems in view of the quickness feature of the auto-focus system. In the object side lens feed system, with a zoom lens system of a variable length a reduction of the object distance increases the difference of feed extent due to the focal distance, thus giving rise to the problems as noted above.
As a method of solving the above problems, there is known system, in which the transverse magnification of a focusing lens unit is changed with zooming such that the extent of feed of lens units at the time of focusing is fixed irrespective of the focal distance. An example of such zoom lens is disclosed in Japanese Patent Laid-Open Application Sho 58-202416. In this disclosed lens system, for focusing three focusing lens units are displaced independently of zooming and in unison with one another substantially for a fixed extent irrespective of the focal distance.
As an entirely difference method, Japanese Patent Laid-Open Application Sho 57-4018 discloses a structure, in a novel focusing cam for interlocking a zooming mechanism and a focusing mechanism to each other is provided to permit focusing structurally irrespective of change in the feed extent with changes in the focal distance.
However, in the method noted above, in which the extent of feed for focusing is substantially constant irrespective of changes in the focal distance, i.e., changes in the state of zooming, a plurality of lens units are displaced in unison with one another for focusing. Therefore, it is impossible to adopt a floating mechanism for suppressing close aberration variations, i.e., vary relative air distances of the individual focusing lens units to one another at the time of focusing, that is, it is impossible to maintain high performance when close focusing is done.
In the method using a novel focusing cam for interlocking the zooming and focusing mechanism to each other, curves as shown in FIG. 25, obtained as intersections curved surfaces representing the extent .DELTA.x of feed of focusing lens unit as a function of the inverse 1/F of focal distance and object distance R and planes parallel to a 1/F-.DELTA.x plane and corresponding to various values of R, are displaced in parallel displacement along the 1/F axis to obtain a single substitute curve, which is utilized as focus cam.
In this method, focusing can be obtained even if the extent of feed of the focusing lens unit varies with the focal distance. However, since curves corresponding to various values of R (object distance) are displaced by parallel displacement for substitution by a single curve, it is difficult to realize focusing in case when the extent of feed is not changed simply with the focal distance.
Further, where the floating for suppressing the close aberration variations is adopted for the focusing lens unit, the extent of parallel displacement in the stage of obtaining the focus curve from the individual curved surfaces is no longer fixed, and hence no correspondence between the extent of parallel displacement and focus cam for lens unit can be obtained. Besides, the extent of feed by floating is not presumably changed simply due to aberrations, and it is liable that a focus cam fails to be realized for each focusing lens unit.